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What is the Significance of Slump of Fresh Concrete?

Slump test is most widely used test for checking workability of fresh concrete. Though we use this test to measure workability actually it does not define workability of concrete. The slumping events are classified as

-True Slump
-Shear Slump

When unsupported concrete after lifting mould, slumps uniformly all around i.e. the area of top surface of mould is equal to top surface of unsupported concrete after slump. This concrete is said to be truly slumped.

Different types of slump

When one half of unsupported concrete (Frustum of cone) slides down along inclined plane, shear slump of concrete is considered to be occurred. In this case the test is recommended to be repeated. If still shear slump continues; we can define concrete as harsh mix i.e. mix have lack of cohesion.

When concrete mix have tendency of harshness in addition to being a lean mix, the slump can easily be changed to shear slump or collapse. The collapse slump is as shown in figure i.e. the mix cannot hold any shape but turned into collapse.

Concrete mixes having different workability show no variation at relatively dry range. Mixes having stiff consistency show zero slumps. Rich mixes show satisfactory behavior i.e. under slump test significant sensitivity found with variation of workability. When lean mixes having significant harshness are tested different slump values are found for different samples of the same mix. For different workability approximate magnitude of slumps are provided in following tables:
Workability corresponding to slumps of concrete

European Classification (ENV 206):

Workability corresponding to slumps of concrete (European Classification)
With different content/type of aggregate (especially for fine aggregate), same slump do not means same workability i.e. with same slump different workabilities of concrete mix are observed. This is the justification of statement “Slump does not define workability”. It also not define ease of compaction as compaction is a external effort to reduce void; but slump occur under self weight of sample (concrete).

It does not provide us behavior of fresh concrete under dynamic agitation like vibration, pumping, finishing or falling down through tremie. The significance of slump test is as follows:

-This test helps us to check hour-to-hour or batch-to-batch variation in materials that are fed into mixing machine.

-Increase of this value may guide us about moisture content of the aggregate i.e. increase in slump means increase in moisture content of constituents materials.

-Decrease in this value give us warning to check grading of aggregate like excess in fine aggregate.

-Too low or too high slump like above example warn an mixer operator about problem and help him to take necessary action

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